What is a spectrogram for?

To see visually where the energy of the sound is: bass, mids, highs and possible problems.

Can I tell if an MP3 is low quality?

Often yes: heavily compressed MP3s usually show a cut in the high frequencies.

Not to look at it; with practice it helps you understand your mixes better.

Online audio tool

Look inside your audio: the spectrogram shows which frequencies play over time, useful for reviewing mixes, noises or cuts.

Target search: spectrogram online

Upload your song or audio.

Nivelab draws the spectrogram with the frequency axis.

Identify bass, highs, noises or cuts.

Use it to decide what to fix in your mix or master.